Apparatus for producing a region free from interfering magnetic fields

ABSTRACT

Magnetic field generating coils are arranged at opposite sides of a region to be made free of interference magnetic fields along each of the three coordinate axes. A set of field sensing probes or transducers are also provided along the respective axes with compensation coils to prevent interaction with the field generating coils arranged along the other axes. As a further aspect, undesired interaction of the probes with control circuits of other equipment or devices being used in the region free from interference fields is suppressed by installing at each of the probe locations one or more compensating coils for nullifying the effect of these fields on the probes. Optionally, control amplifiers may be manually adjusted to compensate for fixed fields generated by other equipment being used in the region.

United States Patent 4 1w:

Griese et al.

1 Apr. 2, 1974 APPARATUS FOR PRODUCING A REGION FREE FROM INTERFERINGMAGNETIC FIELDS [75] Inventors: Alfons Griese, Rommelsbach; Alfons A.Kalisch; Hans G. Luz, both of Reutlingen, all of Germany [73] Assignee:Institut Dr. Friedrich Forster,

. Prufgeratebau, Reutlingen,

Germany 22 Filed: Sept. 15,1972

21 App1.No.: 289,257

Primary ExaminerL. T. Hix Attorney, Agent, or Firm-George J. Netter,Esq.

[57] ABSTRACT trol circuits of other equipment or devices being used 52U.S.Cl. ..317/157.5 in the regim free from interference fields is P 51Int. Cl. ..n01r13/00 Pressed by installing at each P lcatins ['58] Fieldof Search .1 317/123 157.5 cmpensating the fect of these fields on theprobes. Optionally, control [56] References Cited amplifiers may bemanually adjusted to compensate for fixed fields generated by otherequipment being UNITED STATES PATENTS I used in the region v 2,697,18612/1954 Anderson 317/1575 11 Claims, 3 Drawing Figures fiffldffii/A y /71 QMB c/ecu/r 32 2 y 29 3Q f'iff -17 t.

Z0 V 1 Z pzoa'ges/A/g m M anew/7- 24 flzaa fisw J a'zau/r PAIENTEDAPR21914 SHEET 2 OF 3 APPARATUS FOR PRODUCING A REGION FREE FROMINTERFERING MAGNETIC FIELDS FIELD OF THE INVENTION within whichmagnetically sensitive devices may be operated.

There are many electrical apparatus having a high sensitivity to ambientmagnetic fields and which, if not compensated for in some manner,severely influence the apparatus operation. For example, modern electronmicroscopes have a very high resolution which under ideal conditions canapproach the theoretical limit of 2.3 Angstroms where by idealconditions is meant that the region within which the electron microscopeis operated is substantially free from all interfering externallygenerated magnetic fields, even the magnetic field of the earth. Theimportance of this will be appreciated when it is noted that magneticfields as low as 0.5-1 millioersteds produce detectable deflections inthe electron beam of an electron microscope. With such apparatusconstant interference fields will only produce a displacement of theimage and can be compensated for as long as the relative orientation ofthe interference field and the microscope are maintained unchanged.However, a more serious problem is created when the interference fieldis alternating, in that it will affect image definition and in that waythe ultimate resolution obtainable by the microscope.

Moreover, it has been found that considerable distortion in color isobtained in color television tubes when the electron beam is shifted byeven a very small amount, and, for this reason, those involved inresearch and development of such picture tubes require areas withinwhich to work with the tubes that are free from external magneticfields. Similarly, spectroscopes must be free from interference fieldsfor optimal perform:

ance.

- One method of suppressing interference fields for relatively largeenclosures in the past has been by shielding the regions againstexternal fields. This was done by enclosing the area with several layersof a material such as mu-metal or some other highly conductive metal.

. However, although this approach is satisfactory for many situations,the cost can be objectionably high, particularly where the shieldedregion is relatively large.

Another known technique for achieving a field-free region is to arrangeindividual field generating coils, such as Helmholtz coils, in the X, Yand Z coordinates encompassing the region. In addition, separate fieldsensing probes are arranged along the same X, Y and Z coordinates fordetecting the presence of interfering magnetic fields and controllingassociated power circuits to the field generating coils for producing afield counter to the interference field. That is,-the apparatus inaccordance with this technique senses the presence of an interferingmagnetic field and an oppositely directed field of the same magnitude isgenerated thereby bringing the resultant field within the controlledregion to zero.

Although the counter field technique just described is satisfactory, ithas several serious drawbacks. First of all, the field sensing probesmust be located sufficiently far from the monitored region to preventinteraction with the compensating coils on the probes. That is, theprobes are not measuring the field within the magnetic free regionalone, but a larger space that includes the region. Moreover, although aremote location of the sensors can be tolerated for relativelyhomogeneous interference fields, such as the magnetic field of theearth, this may not be possible where the fields are generated by suchthings as motors, generators, or electric current conducting lines, forexample. Moreover, to operate satisfactorily, it is necessary that theprobe signals and associated circuitry driving the compensating coils bevery stable since any change in amplification of any one of the probechannels could result in a severe unbalance in the system. Finally, ifthe space or region to be maintained interference free is to bemonitored continuously, it is necessary in the practice of thistechnique that special probes be installed within the space or region,which is a disadvantage.

It is, therefore, a primary object and aim of the present invention toprovide apparatus for establishing a region or working space that isfree from interfering magnetic fields, all of which is obtainedinexpensively and reliably.

A further object is the provision of apparatus for producing a magneticfield free region having field counteracting means producing a resultantzero field within the region even where the operational characteristicsof the various apparatus component elements vary within broad limits.

Another object is the provision of apparatus for producing a field freeregion as in the above objects in which monitoring of interferencefields within the region can be accomplished relatively easily andinexpensively.

A still further object is the provision of apparatus for creating aregion free from interfering magnetic fields in which field sensorsinclude compensation coils to obviate interaction with counter-fieldgenerating coils.

In the practice of the present invention, magnetic field generatingcoils are provided, arranged at opposite sides of the region to be madefree of magnetic field and along each of the three coordinate axes. Aset of field sensing probes or transducers are provided havingcompensation coils to prevent interaction with the field generatingcoils arranged along other axes.

As a further aspect, undesired interaction of the probes with controlcircuits of other equipment or devices being used in the region freefrom interference fields is suppressed by installing at each of theprobe locations one or more compensating coils which nullifies theeffect of these fields on the probes.

'In' yet another aspect of the invention, means are provided formanually biasing control amplifiers to compensate for fixed fieldsgenerated by other equipment being used in the region.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows in schematic form the apparatusof the present invention illustrated particularly for the elimination ofinterfering magnetic fields in a region including an electronmicroscope.

FIG. 2 discloses a system similar to FIG. 1 including a modified probe.

. 3' FIG. 3 depicts a means for producing dynamic control for either ofthe versions of FIGS. 1 or 2.

DESCRIPTION OF PREFERRED EMBODIMENTS Turning now to the drawings andparticularly FIG. 1, there is depictedlin schematic form the circuitapparatus and coil arrangement of the subject invention for providing aregion substantially completely free from interfering, externallygenerated magnetic fields. More particularly, theregion is seen toinclude, for illustrative purposes only, in its central portion anelectron microscope vacuum cylinder and a substantial volume immediatelyadjacent thereto. The region being monitored and treated by theapparatus to be described is encompassed by a three-dimensional set ofHelmholtz coils 11, including pairs of coils l2, l3, l4, l5, l6 and 17,each pair aligned in one of the X, Y and Z coordinate directions. Thatis, with reference to the coordinate axes diagram, the coils 12 and 13,when energized will provide a field parallel to the X-axis, coils 14 and15 parallel to the Y-axis, and coils 16 and 17 parallel to the Z-axis.

In addition, a set of 'X, Y and Z oriented magnetic field sensing probes18, 19 and 20 are located within the region to be maintained free frominterference fields and closely adjacent the cylinder 10. Preferably,each of the probes 18-20 can include a magnetometer, e.g., a flux-gatemagnetometer which generates a signal of value related to the strengthof the magnetic field'existing in the direction of the respective probeaxis. Signals from each of the probes 18-20 are connected via leads 21,22 and 23, respectively, to processing circuits 24, 25 and 26 forproducing signals generally proportional to the probe signals Theprocessing circuits are connected to driving amplifiers 27, 28 and '29,the outputs of which are fed 'overleads 30, 3 1 and 32, to the pairs ofHelmholtz coils for generating fields within the region opposite to thatdetectedby the probes. For simplicity'of illustration, the Helmholtzcoils 12-17 have been shown as comprising a single turn, however, it isto be understood that in a practical embodiment of the invention, suchcoils willusually each comprise a considerably larger number ofwindings.

The term Helmholtz, as applied to the various magnetic field generatingcoils of this invention, refers to the factthat the coils of each pair,i.e., l2 and 13 for the X-axis, are maintained spaced at a distancesubstantially equal to the effective radius of the coils. It can beshown that with such an arrangement the magnetic field so generated ishighly uniform throughou the space between the coils. i

Referring to the output from amplifier 27, for example, the leads 30 areseen to provide energizing power touch of Helmholtz coils l2 and 13 suchthat the magnetic field generated therein will have a resultant field in'a' single direction normal to the planes ofjthe two fields and parallelto the X-axis. In a similar manner, closed loop energization of thefield generating coils for the ,Y- and Z- axes is also available. It canbe shown that if the probes are maintainedrelatively close to thecylinder 10, the magnetic field sensed by the probes will besubstantially thesame as that in the vacuum cylinder 10. f t

Each of the pairs of the field generating coils, its associatedprocessing circuit and driver amplifier, form a closed loop system.Using'the X system as an illustration, if no interfering field ispresent, and thus none sensed by the X probe 18, no signal is applied tothe processing circuit 24 or to the amplifier 27, so that the X-axisHelmholtz coils l2 and 13 will not be energized. On the other hand, whenan interference magnetic field is detected by the X probe 18, a signalof polarity corresponding to the direction of the interference fieldwill be applied to the processing circuitry 24, which will produce atthe output lines 30 of the amplifier 27 a driving current applied to.the Helmholtz coils 12 and 13 of such magnitude and in such directionas to direct a magnetic field in the region of the vacuum cylinder 10opposite to that of the interference field. Specifically, the resultantof the interference field with that generated by coils l2 and 13responsive thereto, is ideally zcro. Operation for the other axes is thesame. In the usual case, an-interfercncc field will not be directedexactly along any of the orthogonal axes, but rather at an angletheretosuch that more than one probe is 'affected by the respectiveinterference field component.

It is important to note that changes in sensitivity of the probes,amplification of the driver amplifiers, or field generating coilefficiency have practically noinfluence on the resultant operation ofthe described apparatus; There must, however, be sufficient overallsensitivity and amplification in each of the closed loops to providecounteraction for the lowest magnitude of interference field that canadversely affect operation of other equipment in the work region. I

Although operation of the FIG. 1 embodiment is generally satisfactory,certain difficulties are encountered in practical operation. First ofall, the magnetic field generating coils of one coordinate axis mayproduce componentfields in the probes of the other coordinate axes,mainly because an accurate arrangement of the probes cannot always bepractically achieved. Also, when the probes are located outside theregion of uniform magnetic field of the Helmholtz coils, the problem isaccentuated. v 1

Another problem arises from the fact that equipment operating within theregion being maintained free from interfering fields, such as anelectron microscope for example, frequently generates a magnetic fieldof its own which is sensed by the probes and interpreted thereby as aninterference field resulting in a further counteracting field beinggenerated in the manner already described. For example, typically, anelectron microscope will produce a magnetic field which at the outersurface of the vacuum cylinder may attain a magnitude as great as 10oersteds. Accordingly, special measures must be taken when suchequipment is operated within the work region to obviate an erroneouscounter field being generated resulting from the detection of theequipment generated field. A particularly effectivetechn'ique for thispurpose and the onedescribed herein, is thev introductionof compensatingwindings onto the probes. v 3

Turning now to FIG. 2 of the drawings, probes'33, 34'

and 35 for monitoring the field condition in a given region, are assumedorientedas inthe FIG. 1 arrangement to detect fields parallel to theX-axis, Y-axis and Z-axis, respectively. As before, conventionalelectric connections are provided from these probes to processingcircuits 36, 37 and 38, the modulation of which are individuallycontrolled in a conventional manner by adjustment of the devices 39, 40and 41. The output of each of the processing circuits 36-38 is fed vialeads 42, 43 and 44 to power or driver amplifiers 45, 46 and 47, whichamplifiers, as in the first described embodiment, generate currentsfunctionally related to the magnetic fields interacting with therespective probes 33-35.

Current from the amplifiers -47 is directed along leads 48, 49 and 50 todrive the X, Y and Z field generating coils 51, 52 and 53, the latterbeing shown schematically as a single turn'each. One lead of each ofpaired leads 48-50 includes a serially arranged resistor 54, 55 and 56.The voltage drops produced in the resistors 54-56 by driving currentsare applied across compensating windings wound on the probes for each ofthe other two coordinate axes. That is, the voltage drop across resistor54 (which is in the X-axis circuit) is applied through a variableresistor 57 to a compensating winding 58 on the X probe, and also via afurther variable resistor 59 to a compensating winding 60 on the Z probe35. Similarly, the voltage across resistor 55 in the circuit to the Ycoil is applied through variable resistor 61 to a compensating winding62 on the Z probe, as well as through a variable resistor 63 to acompensating winding 64 on the X probe. Finally, voltage developedacross-resistor 56 by current powering the Z coils is applied undercontrolof the variable resistor 65 to a compensating winding 66 on the Xprobe and via a variable resistor 67 to a compensating winding 68 on theY probe.

Three other compensation windings for the X, Y and Z probes,respectively, are identified by the numerals 69, 70 and 71, and whichare fed by current from the field current of the equipment being used,such as the field current of the magnetic lense system of an electronmicroscope 72, for example. Threshold adjustment for the windings 69-71is under the individual control of variable resistors 73, 74 and 75,specific adjustment of which will be described.

Operation of the apparatus depicted in FIG. 2 is generally the same asthat in the embodiment of FIG. 1 already described, except thatcompensation of stray field components detected by the probes fromoperation of the different coils is achieved. Initially the threeseparate closed circuits for the X, Y and Z coils are temporarilydisconnected at the lines 42, 43 and 44. Also, at this time, the lenssystems of the electron microscope 72 is turned off and only one of thesets of Helmholtz coils is maintained in operation, the X coil 51, forexample. A prescribed amount of current is caused to flow in the X coil51 and the devices 40 and 41 are referred to to determine if stray fieldeffect is produced in either the Y or Z probes as a result of the Xfield generation. Assuming that there is such an interaction, thenproper adjustment of variableresistors 57 and 59 as well as insuringcorrect polarity by the reversing connections to the terminals 76 and 77if needed, will zero the readings in the devices 40and 41 whereby alleffect of the X coil field on the Y and Z probes is counteracted. Thissame procedure is followed with respect to the other probes bysequential energization of the Y and Z coils. By this techniquewhichcanbe referredto as cross compensation, all effect of componentsfrom the X, Y and Z coil magnetic fields on the probes is eliminated.

For compensating or counteractingany fixed field generated by the,electron microscope 72, the leads 42-44 are temporarily disconnected asbefore. When the electron microscope is switched on, the devices 39-41will experience a deviation from zero as a result of the field producedby the microscope lens system.

Zeroing of the devices 39-41 is accomplished by individual adjustment ofthe variable resistors 73, 74 and 75, and, where needed, reversal ofconnection at terminals 78-80 to produce the correct polarity forrequired compensation.

After initial calibration or compensation for stray fields from theHelmholtz coils and from the equipment being used in the field-freeregion (electron microscope), the leads 42, 43 and 44 are againconnected as shown in FIG. 2. The apparatus is now ready for general useto counteract the effect of interference magnetic fields generated bysources located externally of the region encompassed by the Helmholtzcoils.

In certain circumstances it may be possible to avoid interaction of theprobes with fields generated by the electron microscope by locating theprobes where the effect of the electron microscopes field is minimal.

Another version of the invention for removing the influence ofrelatively constant fields produced by equipment such as an electronmicroscope, is that shown in FIG. 3. As illustrated there, couplingcapacitors 81, 82 and 83 are serially arranged, respectively, betweenthe processing circuits 36-38 and their associated driver amplifiers45-47. In addition, the input of each amplifier 45-47 is connected via amanually adjustable slidewire contact of a resistance potentiometerarranged across a D. C. source: potentiometer 84, amplifier 45;potentiometer 85, amplifier 46; and potentiometer 86, amplifier 47. Theremainder of circuit apparatus can be the same as in FIG. 2.

Manual compensation for the field produced by the electron microscope orother such equipment in the FIG. 3 embodiment is effected by manuallyadjusting each of the slide-wire contacts of the potentiometers 84-86until zero is indicated on each of the devices 39-41. After zeroing inthis manner, the apparatus of the invention is now fully compensated forall constant magnetic fields existing at the location of the probeswhich, although it has been assumed are generated by the electronmicroscope or other equipment, can in actuality be any constant fieldsuch as, for example, the magnetic field of the earthv The capacitors81-83 in conjunction with the input resistance of the respectiveamplifiers 4547 form a time constant as is well known in the electronicarts. It is advisable that the value of these capacitors be chosen inorder that the lower frequency limit be in the range 0.1 Hz, which willexclude slower changes of magnetic field from being balanced out by thesystem. However, since the measuring time required for most electronmicroscope operation is usually below 10 seconds, a frequency limitapproximating the lower frequency limit specified above is mostfeasible, while changes of magnetic field at a higher rate which couldimpair resolution of the elec- I tron microscope will, on the otherhand, be satisfactorily controlled.

What is claimed is:

1. Apparatus for producing a space free from magnetic interferencefields within which space systems sensitive to interference fields arelocated, such as electron microscopes, spectroscopes, and'the like,comprising:

probe means responsive to magnetic fields being located in the space tobe free from interference fields for receiving the sum total of all themagnetic fields existing at the probe means location, said probe meansgenerating electric signals generally proportional to the ambientmagnetic field;

means connected with said probe means for generating an electric currentas a function of the sum total of all the magnetic fields received bythe probe means; and

coil means powered by the electric current for producing furtherelectric fields in directions and of respective magnitudes such that thesum total of all the magnetic fields existing at the probe meanslocation is substantially zer'o.

2. Apparatus as in claim 1, in which the probe means are located in theimmediate vicinity of the systems sensitive to interference fields.

3. Apparatus as in claim 1, in which the probe means are arranged tosense magnetic fields preferentially along the directions of anorthogonal set of coordinates and the coil means are arranged so thatmagnetic fields generated thereby are directed along the field-sensitivedirections of the probe means.

4. Apparatus as in claim 1, in which interaction of the coil means withthe probe means is suppressed by the provision of compensating windingmeans on said probe means, and means providing a current to saidcompensating winding means of such polarity and magnitude as to cancelout the interaction.

5. Apparatus as in claim 1, in which interactions of fields produced bysaid systems sensitive to interference fields with the probe means aresuppressed by compensating winding means wound on said probe means, andfurther means are provided for directing an electric current throughsaid compensating winding means of such magnitude and polarity as tocounteract the systems generated fields.

6. Apparatus for eliminating interference magnetic fields from aprescribed region in which field generating equipment is operated,comprising:

first, second and third coil means arranged along the respective x, yand z orthogonal axes of said region;

first, second and third magnetic'field probe means located within saidprescribed region and each respectively arranged to sense magneticfields along one of the orthogonal axes of said region;

individual processing circuits connected to said probe means forproviding electric signals substantially proportional to the magneticfields sensed by the associated probe means; and

separate amplifying means interconnecting each processing circuit andthe associated coil means for powering said coil means to produce amagnetic field in said region cancelling interference fields sensed bythe probe means.

7. Apparatus as in claim 6, in which each probe .means is provided withcompensating winding means,

and an individually selectively variable source of electric power isconnected to each said compensating winding means for cancelling out ineach probe means the effect of fields generated by the coil meanslocated in the other coordinate axes. I

8. Apparatus as in claim 6, in which each probe means is provided withcompensating winding means, and an individually selectively variablesource of electric power is connected to each said compensating windingmeans for cancelling out in each probemeans the effects of fieldsproduced by said field generating equipment.

9. Apparatus as in claim 6, in which there are provided first and secondcompensating windings on each probe means, and individual selectivelyadjustable electric power sources connected to each compensating windingthe adjustment of which effectively cancels out for each probe meansboth the field effect of coil means in the other coordinate axes andthat of the field generating equipment.

10. Apparatus as in claim 6, in which individual selectively variableelectric power source means are connected to each amplifying means,individual adjustment of which compensates for relatively constantinterference fields.

11. Apparatus as in claim 6, in which capacitor means interconnect eachprocessing circuit with its associated amplifying means so that currentchanges in the coil means are only produced on changes in magneticfields sensed by the probe means.

UNITED STATES. PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 377Dated jpril 2. 1974 Inventor) ALFONS GRIESE, ALFONS A. KALISCH, HANS s.LUZ

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

The following information should be inserted on the title page} [30]Foreign Application Priority Data Sep. 15, 1971 Germany .2l4607l.9

(SEAL) Attest:

MCCOY M. GIBSON, JR. Attesting Officer 0. MARSHALL DANN Commissioner ofPatents F ORM PO-IOSO (IO-69) I u.s. covznuuaur PRINTING OFFICE an0-36-384,

Signed and sealed this 30th day of July 1974.

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. .3 f301 8'77 Dated April 2 l 1974 Inventofls) ALFONS GRIESE, ALFONS A.KALISCH, HANS G. LUZ

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

The following information should be inserted on the title page;

[30] Foreign Application Priority Data Sep. 15, 1971 Germany ..2l4607l.9

(SEAL) Attest:

MCCOY M. GIBSON, JR. c. MARSHALL DANN Attesting Officer I Commissionerof Patents USCOMM-DC GOS'IQ-PUO FORM PO-OSO (IO-69) u.s. covnuuspumannaomce nu o-au-au.

1. Apparatus for producing a space free from magnetic interferencefields within which space systems sensitive to interference fields arelocated, such as electron microscopes, spectroscopes, and the like,comprising: probe means responsive to magnetic fields being located inthe space to be free from interference fields for receiving the sumtotal of all the magnetic fields existing at the probe means location,said probe means generating electric signals generally proportional tothe ambient magnetic field; means connected with said probe means forgenerating an electric current as a function of the sum total of all themagnetic fields received by the probe means; and coil means powered bythe electric current for producing further electric fields in directionsand of respective magnitudes such that the sum total of all the magneticfields existing at the probe means location is substantially zero. 2.Apparatus as in claim 1, in which the probe means are located in theimmediate vicinity of the systems sensitive to interference fields. 3.Apparatus as in claim 1, in which the probe means are arranged to sensemagnetic fields preferentially along the directions of an orthogonal setof coordinates and the coil means are arranged so that magnetic fieldsgenerated thereby are directed along the field-sensitive directions ofthe probe means.
 4. Apparatus as in claim 1, in which interaction of thecoil means with the probe means is suppressed by the provision ofcompensating winding means on said probe means, and means providing acurrent to said compensating winding means of such polarity andmagnitude as to cancel out the interaction.
 5. Apparatus as in claim 1,in which interactions of fields produced by said systems sensitive tointerference fields with the probe means are suppressed by compensatingwinding means wound on said probe means, and further means are providedfor directing an electric current through said compensating windingmeans of such magnitude and polarity as to counteract the systemsgenerated fields.
 6. Apparatus for eliminating interference magneticfields from a prescribed region in which field generating equipment isoperated, comprising: first, second and third coil means arranged alongthe respective x, y and z orthogonal axes of said region; first, secondand third magnetic field probe means located within said prescribedregion and each respectively arranged to sense magnetic fields along oneof the orthogonal axes of said region; individual processing circuitsconnected to said probe means for providing electric signalssubstantially proportional to the magnetic fields sensed by theassociated probe means; and separate amplifying means interconnectingeach processing circuit and the associated coil means for powering saidcoil means to produce a magnetic field in said region cancellinginterference fields sensed by the probe means.
 7. Apparatus as in claim6, in which each probe means is provided with compensating windingmeans, and an individually selectively variable source of electric poweris connected to each said compensating winding means for cancelling outin each probe means the effect of fields generated by the coil meanslocated in the other coordinate axes.
 8. Apparatus as in claim 6, inwhich each probe means is provided with compensating winding means, andan individually selectively variable source of electric power isconnected to each said compensating winding means for cancelling out ineach probe means the effects of fields produced by said field generatingequipment.
 9. Apparatus as in claim 6, in which there are provided firstand second compensating windings on each probe means, and individualselectively adjustable electric power sources connected to eachcompensating winding the adjustment of which effectively cancels out foreach probe means both the field effect of coil means in the othercoordinate axes and that of the field generating equipment. 10.Apparatus as in claim 6, in which individual selectively variableelectric power source means are connected to each amplifying means,individual adjustment of which compensates for relatively constantinterference fields.
 11. Apparatus as in claim 6, in which capacitormeans interconnect each processing circuit with its associatedamplifying means so that current changes in the coil means are onlyproduced on changes in magnetic fields sensed by the probe means.